In preparing a wet seal to be applied to the edges of a substrate it has been routine to select a particular relatively large particle size for the powder filler used in the impregnation or coating that forms the seal. It has been found that this does not allow adequately dense packing to assure a relatively low porosity. Not only does leakage of the gaseous fuel or oxidant occur but the seal may be still somewhat compressible when the cell components are assembled and may not seal securely against the associated separator plate and associated elements.